1. Technical Field
The present disclosure relates to systems and methods for providing energy to biological tissue and, more particularly, to systems and methods for precisely sensing thermal parameters of tissue during a microwave ablation procedure.
2. Background of Related Art
Energy-based tissue treatment is well known in the art. Various types of energy (e.g., electrical, ultrasonic, microwave, cryogenic, thermal, laser, etc.) are applied to tissue to achieve a desired result. Electrosurgery involves application of high radio frequency electrical current to a surgical site to cut, ablate, coagulate or seal tissue. In monopolar electrosurgery, a source or active electrode delivers radio frequency energy from the electrosurgical generator to the tissue and a return electrode carries the current back to the generator. In monopolar electrosurgery, the source electrode is typically part of the surgical instrument held by the surgeon and applied to the tissue to be treated. A return electrode is placed remotely from the active electrode to carry the current back to the generator. In tissue ablation electrosurgery, the radio frequency energy may be delivered to targeted tissue by an antenna or probe.
There are several types of microwave antenna assemblies in use, e.g., monopole, dipole and helical, which may be used in tissue ablation applications. In monopole and dipole antenna assemblies, microwave energy generally radiates perpendicularly away from the axis of the conductor. Monopole antenna assemblies typically include a single, elongated conductor. A typical dipole antenna assembly includes two elongated conductors, which are linearly aligned and positioned end-to-end relative to one another with an electrical insulator placed between them. Helical antenna assemblies include a helically-shaped conductor connected to a ground plane. Helical antenna assemblies can operate in a number of modes including normal mode (broadside mode), in which the field radiated by the helix is maximum in a perpendicular plane to the helix axis, and axial mode (end fire mode), in which maximum radiation is along the helix axis. The tuning of a helical antenna assembly may be determined, at least in part, by the physical characteristics of the helical antenna element, e.g., the helix diameter, the pitch or distance between coils of the helix, and the position of the helix in relation to the probe assembly to which the helix is mounted.
The typical microwave antenna has a long, thin inner conductor that extends along the longitudinal axis of the probe and is surrounded by a dielectric material. An outer conductor surrounds the dielectric material and extends along the axis of the probe. In another variation of the probe that provides effective outward radiation of energy or heating, a portion or portions of the outer conductor are selectively removed. This type of construction is typically referred to as a “leaky waveguide” or “leaky coaxial” antenna. Another variation on the microwave antenna probe involves having the tip formed in a uniform spiral pattern, such as a helix, to provide the necessary configuration for effective radiation. This variation can be used to direct energy in a particular direction, e.g., perpendicular to the axis, in a forward direction (i.e., towards the distal end of the antenna), or any combination of these directions.
Invasive procedures and devices have been developed in which a microwave antenna probe is either inserted directly into a point of treatment via a normal body orifice or percutaneously inserted. Such invasive procedures and devices potentially provide better temperature control of the tissue being treated. Because of the small difference between the temperature required for denaturing malignant cells and the temperature injurious to healthy cells, a known heating pattern and predictable temperature control is important so that heating is confined to the tissue to be treated. For instance, hyperthermia treatment at the threshold temperature of about 41.5° C. generally has little effect on most malignant growth of cells. However, at slightly elevated temperatures above the approximate range of 43° C. to 45° C. or with rapid bursts of elevated temperatures, thermal damage to most types of normal cells is routinely observed. Accordingly, great care must be taken not to exceed these temperatures along the length of the ablation probe when it is placed adjacent to healthy tissue.
In the case of tissue ablation, a high radio frequency electrical current in the range of about 500 MHz to about 10 GHz is applied to a targeted tissue site to create an ablation volume, which may have a particular size and shape. The ablation volume is correlated to antenna design, antenna tuning, antenna impedance and tissue impedance. Tissue impedance may change during an ablation procedure due to a number of factors, e.g., tissue denaturization or desiccation occurring from the absorption of microwave energy by tissue. Changes in tissue impedance may cause an impedance mismatch between the probe and tissue, which may affect delivery of microwave ablation energy to targeted tissue. The temperature and/or impedance of targeted tissue, and of non-targeted tissue and adjacent anatomical structures, may change at varying rates, which may be greater, or less than, expected rates. A surgeon may need to perform an ablation procedure in an incremental fashion in order to avoid exposing targeted tissue and/or adjacent tissue to excessive temperatures and/or denaturation. In certain circumstances, a surgeon may need to rely on experience and/or published ablation probe parameters to determine an appropriate ablation protocol (e.g., ablation time, ablation power level, and the like) for a particular patient.
One way to monitor and control the temperature of tissue during a tissue ablation procedure is to provide feedback from a tissue sensor probe. If, however, the tissue sensor probe is positioned near a microwave ablation probe, the electromagnetic fields produced by the microwave ablation probe may reduce the accuracy and precision of the measurements of the tissue sensor probe either through direct thermal agitation of the thermal physics of the tissue sensor probe or through induced electrical current in the tissue sensor probe. One solution to this problem is to maintain an appropriate distance between the tissue sensor probe and the electromagnetic fields produced by the microwave field during a surgical procedure. It is difficult, however, to predict the location and boundary of this electromagnetic field. In addition, electromagnetic energy from other sources may interfere with the operation of the tissue sensor probe.